Hybrid motors have recently been given greater attention in the space community. Hybrid rocket motors use reactants of different physical phase states, usually a solid fuel such as rubber and a gaseous oxidizer, such as nitrous oxide. Hybrid motors do not generally deliver the performance of liquid motors. However, hybrid motors are safer and simpler to build and to operate. Hybrid motors can have good performance but often have problems maintaining the proper fuel to oxidizer ratio over the duration of the burn. Hybrid motors disadvantageously also tend to be physically long along the rocket motor axis for the same reasons. Hybrid motors can have complicated systems for introducing the gaseous oxidizer portion at different positions length-wise in the fuel section.
Hybrid solid fuel bodies are generally two-dimensional shapes extruded into the third dimension, for a simple example, a thick-walled tube extruded along the length of the tube. Such a tube is characterized as having a center axial flow channel. The oxidizer is injected through an intake opening and into the solid fuel body and out through a nozzle as exhaust. The fuel is ignited by an igniter positioned proximal to where the oxidizer first contacts the fuel near the intake. The solid fuel bodies generally have a center elongated flow channel through which the oxidizer flows after ignition for ablating the fuel on the side walls of the center elongated flow channel. The fuel is burned on the internal surface effectively ablating the solid fuel interior walls. As the fuel is burned, the combustion becomes oxidizer rich. Oxidizer rich burning provides poor burning efficiency of the solid fuel. Complex fuel grain shapes are sometimes used to increase the amount of surface area in the elongated center flow channel, but sometimes at the risk of an unsupported section of fuel breaking off and plugging the nozzle, causing a catastrophic failure of the hybrid motor. As the fuel burns through the elongated center flow channel, the oxidizer burns the inside of the channel. The growing diameter of the elongated center flow channel changes the ratio between the oxidizer flowing in the channel and the exposed burning fuel on the side walls of the elongated center flow channel. The hybrid rocket motor suffers from changing oxidizer to fuel ratio. The oxidizer to fuel ratio becomes oxidizer rich and thereby wastes available oxidizer that could otherwise be used for more burning of the fuel.
Another problem that is associated with hybrid motors, at least for use in launch vehicles, is low regression rates, typically one third of that of composite solid propellants. Regression rate is the depth-wise rate at which the fuel is removed from the surface where burning occurs. This is a factor in the development of rocket engine thrust. A great amount of research has gone into replacing the solid rocket boosters on the Space Shuttle with hybrid motors only to show that hybrids suffer from low regression rates, which may make replacing large solid motors very difficult. Increased surface area could alleviate this problem.
Stereolithography is a well-known method of building three-dimensional shapes. Stereolithography is generally regarded as a rapid prototyping tool and is typically used to create mock-ups or models for checking the fit, function, and aesthetics of a design. Stereolithography is planar lithographic layering process for building of a three-dimensional solid. Stereolithography uses a platform or substrate that is repeatedly immersed in a photopolymer bath. The exposed photopolymer surface is processed one layer at a time effectively adding many patterned layers upon each other in turn. The light from a moving laser beam exposes and cures the thin two-dimensional layer of photopolymer. With each successive immersion, a new layer of photopolymer is added and a three-dimensional overall shape is eventually made.
There are several rapid prototyping techniques. Stereolithography uses a photopolymer and a curing mechanism. Fused deposition and 3D printing modeling rapid prototyping processes melt plastic and inject the plastic through a moving nozzle or lay down a field of granules, which are selectively bonded together with a binding agent or sintered together with a powerful laser heat source. In all cases, a three-dimensional form is created under computer control by building up two-dimensional layers.
A relatively small cost is required for added design complexity using stereolithography because no dedicated physical tooling is required for this process. For example, a complex buried helical path, for example, can be fabricated, as well as simple straight paths and channels. Many other desired features can be fabricated in the solid using stereolithography, including pitted, rutted, or undulating surfaces made from a plurality of photopolymer layers. The plastic part fabricated using stereolithography have been prototypes unsuitable for space usage.
U.S. Pat. No. 5,367,872, entitled A method and apparatus for enhancing combustion efficiency of solid fuel hybrid rocket motors, issued Nov. 29, 1994 to Lund and Richman, teaches a hybrid rocket motor using a plurality of axially aligned fuel grains having multiple axial perforations. Lund claims a hybrid rocket motor comprising a combustion chamber having aft and forward sections, and a plurality of solid fuel grains. Each grain contains more than one perforation. The fuel grains are cartridge loaded into the combustion chamber along a rocket motor axis such that the perforations of at least two solid fuel grains are misaligned with the perforations of an adjacent solid fuel grain so that the fuel grains are arranged within the combustion chamber to allow gas flow through the perforations in a direction substantially parallel to the rocket motor axis. The individual cartridges can be rotated within a combusted chamber for aligning the perforations. Lund teaches that the flow direction is substantially parallel to the axial elongated center flow channel. Lund teaches that fuel grain cartridges, which are fuel disks having perforated apertures, can be aligned in a combustion chamber. Each fuel grain cartridge has a plurality of aligned perforations through which oxidizer can flow to burn the solid fuel. The perforations are aligned so that the gas flows in all of perforation channels are parallel to each other, substantially in the same axial directly between an intake and exhaust. When all of the flow through the solid fuel is in the same parallel direction, each of the perforations experience like changing of the oxidizer to fuel ratio creating uneven burning.
Existing hybrid rocket motors suffer from oxidizer rich burning, limited parallel axial flow configuration, limited length of the axial channel burning, collapsing of solid rocket bodies, relatively low regression rates and limited extruded channel shapes. These and other disadvantages are solved or reduced using the invention.